The present invention relates to a golf ball having a good feel on impact, a low spin rate on shots with a driver, and an excellent rebound.
Recent golf balls are predominantly two-piece or multi-piece solid golf balls having a core enclosed by a cover of one or more layer. The aim of providing a golf ball with a multilayer construction is generally to impart the ball with all of the following qualities at the same time: a good feel on impact, a high initial velocity, and an appropriate spin rate or launch angle.
The core material present at the center of a golf ball is the part of the ball of greatest importance for conferring the golf ball with a high initial velocity. It is well known that a high initial velocity can be achieved with the use, in particular, of a core having a polybutadiene-containing rubber composition.
Also, JP-A 2002-126129 (U.S. Pat. No. 6,561,929) reports that, by using a core in which the hardness gradually increases from the center out to the surface of the core, the spin rate of the golf ball when hit can be suppressed. As is apparent from such prior-art publications, the core of a golf ball is known to exert an influence on the spin characteristics of the ball.
However, such cores leave something to be desired in terms of their initial velocity performance.
JP-A 2006-167452 (U.S. Patent Application No. 2006/128900) discloses a golf ball core which, owing to the addition of 0.1 part by weight of sulfur to the base rubber so as to increase the hardness difference between the center and surface portions of the core, has both a spin-suppressing effect and an initial velocity-increasing effect.
Yet, although such art attempts, through the addition of a suitable amount of sulfur to the base rubber, to achieve a core hardness distribution of at least a certain size and thus reduce the spin rate on shots with a driver, a still further improvement in performance is desired.
JP-A 2002-336383 (U.S. Pat. No. 6,623,380) describes art in which elemental copper is added so as to lower the PGA compression value of a rubber core by 5 or more units, thereby softening the rubber material. However, the hardness distribution at the interior of the core is not described. Moreover, because elemental copper itself has a high specific gravity, there is some concern that the core weight will become too heavy.